Method of copying holograms

ABSTRACT

A method of copying holograms wherein the original hologram and the detector are spaced apart at least a couple of wavelengths during the copying so that the undiffracted or 0-order wavefront transmitted by the original hologram can serve as the reference beam thus eliminating the need for introducing a separate beam.

OR 39758n186 United States 1 3 3 we) Brumm l Sept. 11, 1973 METHOD OFCOPYING HOLOGRAMS A. 1952, pp. l9322l (pp. 206-209, 2l7 relied upon)[75] lnvcmor: Douglas Brumm Ann Arbor. Harris et 211., Applied Optics,Vol 5. No. 4, April I966,

Mich. pp. 665666 Assigneel The aanfue Development Primary Examinr-DavidSchonberg Corporation, Columbus, Ohio Assistant Examiner-Ronald J. Stern[22] Filed: 30 1966 Att0rneyW00dC0ck, Washburn, Kurtz and a Mackiewicz[21] Appl. No.: 598,008

[57] ABSTRACT [52] U.S. Cl. 350/35, 355/2 A method of copying hologramswherein the original [51] Int. Cl. G02b 27/00 hologram and the detectorare spaced apart at least a [58] Field of Search 350/35; 355/2 couple ofwavelengths during the copying so that the undiffracted or O-orderwavefront transmitted by the [56] References Cited original hologram canserve as the reference beam thus OTHER PUBLICATIONS Rogers, Proc. ofRoyal Soc. of Edinburgh, Vol. 63, Sec.

eliminating the need for introducing a separate beam.

2 Claims, 4 Drawing Figures 1 METHOD OF COPYING HOLOGRAMS This inventionrelates to the production of copies from a hologram and has for anobject the provision of a method using either the real or virtual imageor both reconstructed from a hologram as an object for a secondhologram, with the undiffracted or zero-order wavefront being used asthe reference beam. The present invention permits the first hologram tobe copied without requiring close contact between the two emulsionsinvolved.

it is frequently necessary to make a large number of copies from anoriginal hologram. Heretofore the methods of duplicating or reproducingholograms have utilized contact printing techniques. In employing suchtechniques, if a high resolution photographic emulsion is placed veryclose, i.e., within a few microns, to the original hologram emulsion andthen exposed to a light source, subsequent photographic development willproduce a copy of the original hologram which will reconstruct an imageof the original object when illuminated by a source of monochromaticspatially coherent light. The contact printing method requires that thetwo emulsions involved be close together during the exposure in order toprevent a loss of resolution due to diffraction of the incident light bythe original hologram. lf close contact is not maintained during theexposure, diffraction in the region between the two emulsions willseriously deteriorate the fringe pattern recorded by the emulsion of thecopy. This yields a hologram from which a good reconstruction cannot beobtained. The difficulty of obtaining sufficiently close contact betweenthe two emulsions is the major disadvantage of the contact printingmethod of copying holograms. Holograms have been copied by such methodsbut specialized equipment is needed, such as a vacuum frame or aprinting frame in which the film can be pressed tightly against thehologram emulsion. Such methods have also required the flexibility offilm as distinguished from the use of plates.

In accordance with the present invention, a hologram copy is produced bygenerating a new fringe system rather than by duplicating the fringes ofthe original hologram. This new fringe pattern is not the same as theoriginal fringe pattern but it does reconstruct an image of the originalobject which is the desired result. This is achieved by illuminating theoriginal hologram with a monochromatic spatially coherent light such as,for example, a monochromatic plane wave in such a way that a goodreconstructed image is obtained. An unexposed, high-resolution emulsionfilm or plate is then placed behind the hologram in such a way that itintercepts the undiffracted wavefront passing through the hologram aswell as the reconstructed image wavefront which is diffracted or itsconjugate. These wavefronts then interfere to produce a new set offringes. This new fringe system, when properly illuminated, willreconstruct an image of the original object and its conjugate.

For purposes of explanation, it will be assumed that the originalhologram is infinitely thin and that the copy hologram is made whileactually in contact with the original hologram. The wavefront passingthrough the original hologram when it is illuminated with amonochromatic plane wavefront has three components of interest, theundiffracted or zero-order wavefront and the two first-order wavefrontswhich are complex conjugates of one another. These three componentwavefronts interfere in pairs to produce the three components of thetotal fringe system formed on the copy hologram. Only two of thesecomponents are of interest, those formed by interference between theundiffracted wavefront and each of the diffracted wavefronts. Afterphotographic development, each of these component patterns willreconstruct wavefronts corresponding to the original object wavefrontand the conjugate when illuminated with a monochromatic plane wavefront.Thus, there are two reconstructed virtual images and two conjugateimages. For the conditions assumed, however, i.e., two emulsions incontact during exposure of the copy, the two virtual images coincide andsimilarly for their conjugates. Now if the emulsions are separated by adistance d during exposure of the copy, the virtual images and theirconjugates will be separated by 2d in the reconstruction. This imagedoubling will not usually occur in practice, however, as hologram emulsions cannot be infinitely thin. Most holograms will not reconstructboth the virtual image and its conjugate for I the same hologramorientation because of the Bragg effect; i.e., the original hologram canbe oriented to optimize one image or the other, but not bothsimultaneously. Thus, only two wavefront components will interfere inthe copy hologram plane, the undiffracted wavefront and one of thefirst-order wavefronts. Thus, the copy hologram will contain only oneset of fringes and will reconstruct one virtual image and one conjugateimage.

The present method actually involves making a new hologram with theundiffracted wavefront serving as the reference beam and thereconstructed image of the original object or its conjugate taking theplace of an actual object. While previous methods have been proposed formaking a hologram of a hologram, such methods have required theintroduction of a separate reference beam. The present process does notrequire a separate reference beam and no mirrors or beam splitters arerequired. In one of the prior art methods, the real image produced by ahologram was used as an object for a second hologram. Of course, thevirtual image could be used in a similar manner. With such method, usingeither image, it was necessary for the original hologram and theunexposed plate or detector to be far enough apart in order to allow thedetector to intercept only the diffracted wavefront corresponding to thedesired image. This eliminated interference with the undiffractedwavefront and a conjugate diffracted wavefront. This wide separationdistance between the original hologram and the detector was alsorequired to allow introduction of the reference beam.

The present invention does not require a separate reference beam nor thewide separation distance between the original hologram and the detector.Furthermore, it does not require the close contact between the twoemulsions of the original hologram and the detector as required by priorcontact printing processes. In accordance with the present invention,the original hologram and the detector are spaced apart a predetermineddistance sufficiently large so that the undif fracted or zero-orderwavefront transmitted by the original hologram can serve as thereference beam thus eliminating the need for introducing a separatebeam. For example, the spacing should be at least a couple ofwavelengths for the type of copying described here.

More particularly, the present invention provides a method of copying ahologram of an object comprising the steps of placing a hologram and adetector in parallel planes so that the recording surfaces thereof arespaced apart a predetermined distance, directing an illuminating beam ofmonochromatic spatially coherent light onto the recording surface of thehologram to produce a reconstructed image of the original object, usingthe illuminating beam as a reference beam to interfere with the imagewavefront to produce an interference pattern. and recording theinterference pattern on the detector.

In accordance with one aspect of the invention, the reconstructed imageformed by the hologram is produced by light that is diffracted andtransmitted through the hologram to produce a front-beam copy of thehologram.

In accordance with a further aspect of the invention, the reconstructedimage formed by the hologram is produced by light reflected from thehologram to produce a back-beam copy of the hologram.

In accordance with a further aspect of the invention, one interferencepattern is produced from the image wavefront and the illuminating beamwhich is transmitted only through the hologram, and a secondinterference pattern is produced from the image wavefront and theilluminating beam which is transmitted through the detector andreflected back to the detector to produce a composite front-beam andback-beam hologram copy.

In accordance with another aspect of the invention, one interferencepattern is produced from the image wavefront and the illuminating beamwhich is incident on the detector and a second interference pattern isproduced from the image wavefront and the illuminating beam which istransmitted through the hologram and reflected back to the detector toproduce a composite front-beam and back-beam hologram copy.

For a more detailed disclosure of the invention and for further objectsand advantages thereof, reference is to be had to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a diagramatic view of a system for copying front-beamholograms in accordance with the present invention;

FIG. 2 is a diagramatic view of a system for copying back-beam hologramsin accordance with the present invention;

FIG. 3 is a diagramatic view of a system for producing a compositefront-beam and back-beam hologram copy from a front-beam original; and

FIG. 4 is a diagramatic view of a system for producing a compositefront-beam and back-beam hologram copy from a back-beam original.

Wavefront reconstruction using coherent radiation is described in U. 5.Pat. application, Ser, No. 36l,977, filed Apr. 23, 1964 and now US. Pat.No. 3,506,327 and in U. S. Pat. application, Ser. No. 503,993, filedOct. 23, 1965 and now abandoned. The holographic or wavefrontreconstruction process, in its simplest form, includes directing a firstbeam of coherent light onto an object and positioning a detector,usually a photographic plate, to receive the light emanating from theobject. A second beam of light, coherent with respect to the first beam,is directed at some selected angle to the light emanating from theobject and onto the detector causing the light from the object and thelight of the second beam, called the reference beam, to form a patternof interference fringes at the detector which the detector records. Thispattern of interference fringes is referred to in the art as a hologramand may be reconstructed to produce a three-dimensional real image orvirtual image of the original object. The reconstruction occurs when thehologram is illuminated with coherent light. When the hologram isproduced by applying the object bearing beam and the reference beam tothe same side of the detector, the hologram thus produced is referred toin the art as a front-beam hologram. In reconstructing an image of theoriginal object from a front-beam hologram the coherent light isdirected upon the hologram and the image beam is transmitted through anddiffracted by the front-beam hologram. Such a technique is disclosed inthe article entitled Photography by Laser" by Emmett N. Leith and .IurisUpatnieks in Scientific American for June, 1965, pages 24-35.

Holograms may also be produced of the back-beam type wherein the objectbearing beam is directed onto one side of the detector while thereference beam is directed onto the reverse side of the detector.Holograms of the back-beam type are disclosed in co-pending U. S. Pat.application, Ser. No. 538,854, filed Mar. 30, 1966, by Nile F. Hartmann.In reconstructing holograms of the back-beam type, light is directedupon the hologram and the image beam is reflected from the back-beamhologram. Holograms of the back-beam type are also disclosed in ColorHolograms for White Light Reconstruction" by J. Upatnieks, 1. Marks andR. Fedorowicz, Applied Physics Letter, Vol. 8, No. 11 (1966), Page 286.

Referring to FIG. 1, the present invention has been illustrated inconnection with copying a hologram of the front-beam type. The originalhologram H, has been produced in accordance with the aforedescribedfront-beam method and the detector H,, preferably in the form of anundeveloped photographic plate, is positioned a predetermined distancefrom the hologram H, and in a plane parallel thereto. As may be seen inFIG. 1, the recording surfaces R, and R,, which in the case ofphotographic plates are emulsion surfaces, are positioned to the lefthand side of the hologram H, and the detector H,. The predetermineddistance between the recording surfaces R, and R, should be at least acouple of wavelengths and in practice the hologram H, and the detectorH, may be placed in contact with each other so that the recordingsurfaces R, and R, will be separated by a distance corresponding to thethickness of the emulsion backing on hologram H,. For example, with aphotographic plate, such as a Kodak 649F, and with the recording surfaceR, against the backing of the hologram H,, the recording surfaces R, andR, will be separated by a distance of approximately one-eighth inch orone-fourth inch depending upon the type of plate used. It is to beunderstood taht in FIG. 1 the hologram H, and the detector H,, have beenillustrated as separated by a greater distance for purposes of clarityin illustrating the ray diagrams. This is also the case in FIGS. 2-4.

In producing a copy of the front-beam hologram H,, the detector H, ispositioned as shown in FIG. 1 so that the illumination is inclined at anangle so as to produce a bright image from the hologram H,. In practice,

image doubling does not usually occur as the hologram emulsions are notinfinitely thin. Most holograms will not reconstruct both the virtualimage and its conjugate for the same hologram orientation because of theBragg effect; i.e., the original hologram can be oriented to optimizeone image or the other but not both simultaneously. As may be seen inFIG. 1, the illuminating beam I when it strikes the surface R, of thehologram H, is divided into an image beam I, which is diffracted and theundiffracted or zero-order beam I The beam I, produces a reconstructedimage of the original object and, thus, serves as the object beam inproducing the second hologram H,. The illuminating beam I also serves asthe reference beam I to interfere with the image wavefront I, to producean interference pattern which is recorded on the detector or secondhologram H,. Good quality hologram copies have been produced with thearrangement illustrated in FIG. 1 and using light from a helium-neonlaser (6,328 Angstroms).

Referring to FIG. 2, there is illustrated a modification of theinvention wherein an original hologram H, of the back-beam type iscopied on a detector H It will be noted in FIG. 2 that the recordingsurfaces R and R are disposed on the right hand sides of the members H,and H respectively. As in the previous arrangement of FIG. 1, thehologram H and the detector H, are positioned parallel to each other andare spaced apart a predetermined distance which in practice ispreferably the thickness of the backing on the hologram H,. In thisarrangement the original back-beam hologram H is illuminated with a beamI of coherent light which passes through the detector H and thereconstructed image formed by the hologram H; is produced by light I,reflected from the hologram H, to the recording surface R of detector HThe illuminating beam 1 is used as a reference beam to interfere withthe image wavefront I, to produce an interference pattern. It will benoted that the image wavefront I, and the reference beam 1 engage therecording surface R, on opposite sides. Thus the interference patternrecorded on the detector H is of the back-beam type and is referred toas a back-beam copy of the hologram.

In FIG. 3, there is illustrated a method of copying a front-beamhologram H so that the copy H is a composite front-beam and back-beamhologram. As in FIG. 1 the hologram H, is a front-beam hologram and itis in a parallel plane spaced from the detector H The recording surfacesR and R, are both positioned on the left hand sides of the members H andH, respectively. A reflecting surface such as a mirror M is positionedto the right of the detector H,. The front-beam hologram copy on thedetector H, is produced in the same manner as previously described inconnection with FIG. 1. The illuminating beam I engages the recordingsurface R, to produce an image beam I, which carries the reconstructedimage of the original object. The image beam I engages the recordingsurface R, of the detector H, and interferes with the illuminating beamwhich is transmitted through the hologram H; to serve as a referencebeam l to produce an interference pattern on the recording surface R,.This interference pattern is recorded on the recording surface R, as afront-beam hologram. It will also be seen in FIG. 3 that theilluminating beam I also is transmitted through the hologram H, and thedetector H, where it strikes the reflecting surface of the mirror M andis reflected through the detector H, onto the recording surface R Thisreflected beam I, in conjunction with the image beam 1 produces a secondinterference pattern of the backbeam type which is recorded on thedetector H Thus the detecor H, is provided with a composite frontbeamand back-beam hologram copy ofthe original hologram H In FIG. 4 there isillustrated an arrangement for producing a composite front-beam andback-beam hologram copy from an original back-beam hologram. Theoriginal hologram H and the detector H. are positioned in the samemanner as previously described in connection with FIG. 2 and a mirror Mis positioned to one side of the original hologram l-I-,. As in FIG. 2,the back-beam hologram copy is produced by cooperation of theilluminating beam I and the image beam 1 which is reflected from therecording surface R of the original hologram H, to produce aninterference pattern on the recording surface R, of the detector H, Thisinterference pattern is recorded on the detector H, and is of theback-beam type. The illuminating beam also passes through both thedetector H, and the hologram H, where it strikes the reflecting surfaceof the mirror M and is reflected back through the hologram H onto therecording surface R of the detector H, This reflected beam I, acts as areference beam which interferes with the image beam 1 to produce aninterference pattern of the front-beam type which is recorded on thesurface R of detector H Thus it will be seen that the detector H isprovided with a composite front-beam and backbeam hologram copy of theoriginal hologram H From the foregoing, it will be seen that copyholograms can be made without any special contact printing apparatus. Itis only necessary for the original hologram to be illuminated properlyand for the detector, such as a film or plate, to be placed behind theoriginal hologram in such a way that it intercepts the undiffractedwavefront and at least one of the diffracted wavefronts.

It is to be understood the invention is not limited to the specificarrangements shown and that changes and modifications may be made withinthe scope of the appended claims.

What is claimed is:

l. A method of copying an off-axis hologram of the front-beam typecomprising the steps of:

Directing an illuminating beam consisting of monochromatic spatiallycoherent light onto a recording surface of the off-axis front-beamhologram to an angle so as to produce on the opposite side thereof adiffracted image beam at a finite angle with respect to an undiffractedportion of said illuminating beam passing through said recordingsurface,

placing a detector having a recording surface sensitive to coherentlight and capable of detecting interference fringes with respect to thefront-beam hologram so that the recording surfaces thereof are spacedapart at least a couple of wavelengths and so that the detector receivesboth the diffracted image beam and the undiffracted portion of theilluminating beam directly from the recording surface of the hologram,

recording an interference pattern on the detector produced by theinteraction of the undiffracted portion of the illuminating beam and thediffracted image beam as a front-beam copy of the frontbeam hologram,

positioning a reflector on the opposite side of the recording surface ofthe detector from the hologram to receive an undiffracted portion ofsaid illuminating beam passing through the recording surfaces of boththe hologram and the detector for reflection back to the detector, and

recording a second interference pattern on the detector produced by theinteraction of the reflected undiffracted portion of the illuminatingbeam and the diffracted image beam to produce a composite front-beam andback-beam hologram copy.

2. A method of copying an off-axis hologram of the back-beam typecomprising the steps of:

directing an illuminating beam of monochromatic spatially coherent lightonto the recording surface of the off-axis back-beam hologram at anangle so as to produce for reflection therefrom a diffracted image beamat a finite angle with respect to said illuminating beam,

placing in the path of said illuminating beam a detector at least acouple of wavelengths from the recording surface of said hologram toreceive the diffracted image beam reflected from the hologram,

the detector being sensitive to coherent light and capable of detectinginterference fringes, recording an interference pattern on the detectorproduced by the interaction of the illuminating beam and the reflecteddiffracted image beam as a back-beam copy of the back-beam hologram,positioning a reflector on the opposite side of the recording surface ofthe hologram from the detector to receive an undiffracted portion ofsaid illuminating beam passing through the recording surfaces of boththe hologram and the detector for reflection back through the hologramto the detector, and recording a second interference pattern on thedetector produced by the interaction of the reflected undiffractedportion of the illuminating beam and the reflected diffracted image beamto produce a composite front-beam and back-beam hologram copy.

t a a t 23 3 UNH'I'ZD S'IA'II'IS PATENT OFHUI') :3 1 1 w "1 1 w r l@ERTHlCAlE Oi CORRILCJLKOPI Patent; No. 3 I758 I186, Dated September 11,1973 Invcntor(s) glas B. Brumm rs in the abov-idcr1tificd patent:

It is certified that error appca by corrected as shown below:

and that said Letters Patent are here Column 3, line 58, change "and nowabandoned" to -and now 11.5. Pat. No. 3,580,655

Column 4, line 56) correct the spelling of "that".

Column 6 line 4 5, the word "to" should be -a t Signed and sea-led this25th day-of December- 1973;

(SEAL) Attest:

EDWARD M.FLETGHER,JR. RENE D. TEGTMEYER Attesting Officer ActingCommissioner of Patents

1. A method of copying an off-axis hologram of the front-beam typecomprising the steps of: Directing an illuminating beam consisting ofmonochromatic spatially coherent light onto a recording surface of theoff-axis front-beam hologram to an angle so as to produce on theopposite side thereof a diffracted image beam at a finite angle withrespect to an undiffracted portion of said illuminating beam passingthrough said recording surface, placing a detector having a recordingsurface sensitive to coherent light and capable of detectinginterference fringes with respect to the front-beam hologram so that therecording surfaces thereof are spaced apart at least a couple ofwavelengths and so that the detector receives both the diffracted imagebeam and the undiffracted portion of the illuminating beam directly fromthe recording surface of the hologram, recording an interference patternon the detector produced by the interaction of the undiffracted portionof the illuminating beam and the diffracted image beam as a front-beamcopy of the front-beam hologram, positioning a reflector on the oppositeside of the recording surface of the detector from the hologram toreceive an undiffracted portion of said illuminating beam passingthrough the recording surfaces of both the hologram and the detector forreflection back to the detector, and recording a second interferencepattern on the detector produced by the interaction of the reflectedundiffracted portion of the illuminating beam and the diffracted imagebeam to produce a composite front-beam and back-beam hologram copy.
 2. Amethod of copying an off-axis hologram of the back-beam type comprisingthe steps of: directing an illuminating beam of monochromatic spatiallycoherent light onto the recording surface of the off-axis back-beamhologram at an angle so as to produce for reflection therefrom adiffracted image beam at a finite angle with respect to saidilluminating beam, placing in the path of said illuminating beam adetector at least a couple of wavelengths from the recording surface ofsaid hologram to receive the diffracted image beam reflected from thehologram, the detector being sensitive to coherent light and capable ofdetecting interference fringes, recording an interference pattern on thedetector produced by the interaction of the illuminating beam and thereflected diffracted image beam as a back-beam copy of the back-beamhologram, positioning a reflector on the opposite side of the recordingsurface of the hologram from the detector to receive an undiffractedportion of said illuminating beam passing through the recording surfacesof Both the hologram and the detector for reflection back through thehologram to the detector, and recording a second interference pattern onthe detector produced by the interaction of the reflected undiffractedportion of the illuminating beam and the reflected diffracted image beamto produce a composite front-beam and back-beam hologram copy.